LeetCode #1 Two Sum C# Solution

LeetCode #1 Problem
Given an array of integers, return indices of the two numbers such that they add up to a specific target.
You may assume that each input would have exactly one solution.
Example:
Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

基本上看到题第一反应都是一个O(N^2)的双重循环，但是在大数据下必然会TLE，所以考虑其他算法。
可以想到，寻找两个数相加得到target可以转换为target减去ans[0]然后在nums[]中寻找ans[1]是否存在。同时，哈希表拥有查找效率为O(1)的优势，于是这样一重循环就可以找到答案了。

C# Code        public int[] TwoSum(int[] nums, int target)        {            Hashtable ht = new Hashtable();            List<int> list = new List<int>();            list = nums.ToList<int>();            int i = 0;            foreach (int num in list)            {                if (!ht.Contains(num)) ht.Add(num,i);                i++;            }            i = 0;            foreach (int num in list)            {                int temp = target - num;                if (ht.Contains(temp)&&i!=(int)ht[temp])                {                    int[] ans = new int[2];                    ans[0] = i;                    ans[1] = (int)ht[temp];                    return ans;                }                i++;            }            return null;        }